Fix bad reliance on env variable, causing error when interpolating a comma seperated value

lib/less/tree/value.js

@@ -22,7 +22,7 @@ tree.Value.prototype = {
         for(i = 0; i < this.value.length; i++) {
             this.value[i].genCSS(env, output);
             if (i+1 < this.value.length) {
-                output.add(env.compress ? ',' : ', ');
+                output.add((env && env.compress) ? ',' : ', ');
             }
         }
     },

test/css/strings.css

@@ -38,3 +38,6 @@
   color: #000000;
   color: #ffa500;
 }
+.watermark {
+  family: Univers, Arial, Verdana, San-Serif;
+}

test/less/strings.less

@@ -49,3 +49,9 @@
     .mix-mul(black);
     .mix-mul(orange);
 }
+
+@test: Arial, Verdana, San-Serif;
+.watermark {
+  @family: ~"Univers, @{test}";
+  family: @family;
+}